diff --git a/dom/base/CustomElementRegistry.cpp b/dom/base/CustomElementRegistry.cpp
index 9d7e9547f856..bd077d8857a8 100644
--- a/dom/base/CustomElementRegistry.cpp
+++ b/dom/base/CustomElementRegistry.cpp
@@ -103,6 +103,7 @@ NS_IMPL_CYCLE_COLLECTION_CLASS(CustomElementRegistry)
 
 NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(CustomElementRegistry)
   tmp->mCustomDefinitions.Clear();
+  tmp->mConstructors.clear();
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mWhenDefinedPromiseMap)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mWindow)
   NS_IMPL_CYCLE_COLLECTION_UNLINK_PRESERVED_WRAPPER
@@ -150,6 +151,12 @@ NS_IMPL_CYCLE_COLLECTION_TRACE_BEGIN(CustomElementRegistry)
                      "mCustomDefinitions prototype",
                      aClosure);
   }
+
+  for (ConstructorMap::Enum iter(tmp->mConstructors); !iter.empty(); iter.popFront()) {
+    aCallbacks.Trace(&iter.front().mutableKey(),
+                     "mConstructors key",
+                     aClosure);
+  }
   NS_IMPL_CYCLE_COLLECTION_TRACE_PRESERVED_WRAPPER
 NS_IMPL_CYCLE_COLLECTION_TRACE_END
 
@@ -190,6 +197,11 @@ CustomElementRegistry::Create(nsPIDOMWindowInner* aWindow)
 
   RefPtr<CustomElementRegistry> customElementRegistry =
     new CustomElementRegistry(aWindow);
+
+  if (!customElementRegistry->Init()) {
+    return nullptr;
+  }
+
   return customElementRegistry.forget();
 }
 
@@ -248,6 +260,12 @@ CustomElementRegistry::~CustomElementRegistry()
   mozilla::DropJSObjects(this);
 }
 
+bool
+CustomElementRegistry::Init()
+{
+  return mConstructors.init();
+}
+
 CustomElementDefinition*
 CustomElementRegistry::LookupCustomElementDefinition(const nsAString& aLocalName,
                                                      const nsAString* aIs) const
@@ -616,9 +634,13 @@ CustomElementRegistry::Define(const nsAString& aName,
    * 4. If this CustomElementRegistry contains an entry with constructor constructor,
    *    then throw a "NotSupportedError" DOMException and abort these steps.
    */
-  // TODO: Step 3 of HTMLConstructor also needs a way to look up definition by
-  // using constructor. So I plans to figure out a solution to support both of
-  // them in bug 1274159.
+  const auto& ptr = mConstructors.lookup(constructorUnwrapped);
+  if (ptr) {
+    MOZ_ASSERT(mCustomDefinitions.Get(ptr->value()),
+               "Definition must be found in mCustomDefinitions");
+    aRv.Throw(NS_ERROR_DOM_NOT_SUPPORTED_ERR);
+    return;
+  }
 
   /**
    * 5. Let localName be name.
@@ -774,8 +796,16 @@ CustomElementRegistry::Define(const nsAString& aName,
   /**
    * 12. Add definition to this CustomElementRegistry.
    */
+  if (!mConstructors.put(constructorUnwrapped, nameAtom)) {
+    aRv.Throw(NS_ERROR_FAILURE);
+    return;
+  }
+
   mCustomDefinitions.Put(nameAtom, definition);
 
+  MOZ_ASSERT(mCustomDefinitions.Count() == mConstructors.count(),
+             "Number of entries should be the same");
+
   /**
    * 13. 14. 15. Upgrade candidates
    */
@@ -860,4 +890,4 @@ CustomElementDefinition::CustomElementDefinition(nsIAtom* aType,
 }
 
 } // namespace dom
-} // namespace mozilla
\ No newline at end of file
+} // namespace mozilla
diff --git a/dom/base/CustomElementRegistry.h b/dom/base/CustomElementRegistry.h
index 8b2f2e93315a..1df1aed9fb8c 100644
--- a/dom/base/CustomElementRegistry.h
+++ b/dom/base/CustomElementRegistry.h
@@ -7,13 +7,14 @@
 #ifndef mozilla_dom_CustomElementRegistry_h
 #define mozilla_dom_CustomElementRegistry_h
 
+#include "js/GCHashTable.h"
 #include "js/TypeDecls.h"
 #include "mozilla/Attributes.h"
 #include "mozilla/ErrorResult.h"
 #include "mozilla/dom/BindingDeclarations.h"
+#include "mozilla/dom/FunctionBinding.h"
 #include "nsCycleCollectionParticipant.h"
 #include "nsWrapperCache.h"
-#include "mozilla/dom/FunctionBinding.h"
 
 class nsDocument;
 
@@ -172,6 +173,8 @@ private:
   explicit CustomElementRegistry(nsPIDOMWindowInner* aWindow);
   ~CustomElementRegistry();
 
+  bool Init();
+
   /**
    * Registers an unresolved custom element that is a candidate for
    * upgrade when the definition is registered via registerElement.
@@ -191,15 +194,25 @@ private:
     DefinitionMap;
   typedef nsClassHashtable<nsISupportsHashKey, nsTArray<nsWeakPtr>>
     CandidateMap;
+  typedef JS::GCHashMap<JS::Heap<JSObject*>,
+                        nsCOMPtr<nsIAtom>,
+                        js::MovableCellHasher<JS::Heap<JSObject*>>,
+                        js::SystemAllocPolicy> ConstructorMap;
 
   // Hashtable for custom element definitions in web components.
   // Custom prototypes are stored in the compartment where
   // registerElement was called.
   DefinitionMap mCustomDefinitions;
 
+  // Hashtable for looking up definitions by using constructor as key.
+  // Custom elements' name are stored here and we need to lookup
+  // mCustomDefinitions again to get definitions.
+  ConstructorMap mConstructors;
+
   typedef nsRefPtrHashtable<nsISupportsHashKey, Promise>
     WhenDefinedPromiseMap;
   WhenDefinedPromiseMap mWhenDefinedPromiseMap;
+
   // The "upgrade candidates map" from the web components spec. Maps from a
   // namespace id and local name to a list of elements to upgrade if that
   // element is registered as a custom element.
diff --git a/testing/web-platform/meta/custom-elements/CustomElementRegistry.html.ini b/testing/web-platform/meta/custom-elements/CustomElementRegistry.html.ini
index 9e2bf738569b..f62b64dfb547 100644
--- a/testing/web-platform/meta/custom-elements/CustomElementRegistry.html.ini
+++ b/testing/web-platform/meta/custom-elements/CustomElementRegistry.html.ini
@@ -1,8 +1,5 @@
 [CustomElementRegistry.html]
   type: testharness
-  [customElements.define must throw a NotSupportedError when there is already a custom element with the same class]
-    expected: FAIL
-
   [customElements.define must get callbacks of the constructor prototype]
     expected: FAIL
 
diff --git a/testing/web-platform/meta/custom-elements/custom-element-registry/define.html.ini b/testing/web-platform/meta/custom-elements/custom-element-registry/define.html.ini
deleted file mode 100644
index 122abccd31aa..000000000000
--- a/testing/web-platform/meta/custom-elements/custom-element-registry/define.html.ini
+++ /dev/null
@@ -1,26 +0,0 @@
-[define.html]
-  type: testharness
-  [If constructor is HTMLElement, should throw a TypeError]
-    expected: FAIL
-
-  [If constructor is HTMLButtonElement, should throw a TypeError]
-    expected: FAIL
-
-  [If constructor is HTMLImageElement, should throw a TypeError]
-    expected: FAIL
-
-  [If constructor is HTMLMediaElement, should throw a TypeError]
-    expected: FAIL
-
-  [If constructor is Image, should throw a TypeError]
-    expected: FAIL
-
-  [If constructor is Audio, should throw a TypeError]
-    expected: FAIL
-
-  [If constructor is Option, should throw a TypeError]
-    expected: FAIL
-
-  [If the constructor is already defined, should throw a NotSupportedError]
-    expected: FAIL
-